(1) Field of the Invention
The present invention relates to the field of data communications. More particularly the present invention relates to the field of efficient signal transmissions over high speed communication links.
(2) Prior Art
Small differential signals provide a number of advantages for the transmission of high bit rate data signals. A differential channel provides rejection of common mode noise (such as power supply noise) that may be present between a transmitting ("driving") and a receiving node. Differential signals can be transmitted on well known twisted-pair cables which are less expensive than coaxial or fiber optic cables and which when shielded, offer very good rejection of interferences from sources of external noise. Using small signal level differential signals over a shielded twisted-pair cable reduces EMI emissions, simplifies transmitter design, and reduces power dissipation.
One characteristic of a differential communication channel is that it reduces timing distortion due to mismatched rise and fall times and receiver threshold. Timing distortion must be minimized since in a digital communication system data is encoded in both time and amplitude. It is very difficult in a single-ended communication system to match the rise and fall times and this mismatch becomes significant when data rates become high (e.g., 50 Mbaud or more). Differential systems, however, do not suffer duty cycle distortion due to rise and fall time mismatch. Also, within a differential receiver, the threshold is not set externally as in a single-ended system. Instead, the threshold in a differential system is a function of the received signal and therefore tracks with the received signal corresponding to when the differential signal equals zero volts.
As such, it would be advantageous to provide high speed communication using a differential signal configuration. Further, it would be advantageous to provide two signal transmission techniques within a transmitter circuit as defined by the IEEE P1394 standard. The present invention provides for such advantageous functionality.
Communication units of a communication network often communicate at different speeds, depending on their internal configuration and/or depending on their maximum communication capacity. Different units adapted to and coupled to the communication network may communicate at different maximum communication rates. It would be advantageous to provide an efficient method and mechanism for one communication unit to signal its use of a high speed communication rate to another communication unit so that both may utilize a common and proper high speed communication rate. This is especially applicable to a communication network that provides for different communication rates among units of differing versions and complexity. The present invention offers such advantageous capability.
Accordingly, it is an object of the present invention to provide an efficient high speed data communication configuration. It is an object of the present invention to provide the high speed data communication configuration using a differential signal transmission technique that is compatible with the IEEE P1394 standard. It is further an object of the present invention to provide a transmission circuit and method for same that operates in both a differential signal transmission mode and, in addition, operates simultaneously in a common mode transmission mode. Using such a system, it is an object of the present invention to provide a communication unit with the ability to signal its use of a high speed communication rate to another communication unit while minimizing data communication delay. These and other objects not specifically mentioned above will become clear as described further below.